There is a 256 GB 1.8" 5mm LIF SATA-II drive planned to be released late this quarter or early next quarter. This will be from a company that is not currently in the market of selling SSDs for the MacBook Air. Perhaps Runcore & Photofast drives will also compete in that segment of the market.
I often wonder how large the market is for a faster 128 GB SSD in the v 2,1 MBAs? It seems that 256 GB SSDs would be a much more marketable drive considering Apple only sells up to 128 GB SSDs. It has to be difficult for Runcore and Photofast to sell drives that offer no more drive space and only have speed as their advantage. If Apple sticks with 128 GB SSD in their next MBA, these companies will have a much bigger market.
If you want a 256 GB SSD for a v 2,1 MBA, wait a bit and you will probably have multiple options.